Currently, fetal diagnosis is generally carried out by a procedure known as amniocentesis which involves the aspiration of a small sample of amniotic fluid from the pregnant mother, culturing the fetal cells in the fluid, and determining the karyotype of the fetal cells. Recently, chorionic villus sampling has also been used, which involves the direct transcervical and transabdominal aspiration of the chorionic villus. However, as both amniocentesis and chorionic villus sampling require invasive procedures for obtaining fetal cells, they inevitably expose both the mother and the fetus to a certain amount of risk. Accordingly non-invasive approaches to prenatal diagnosis are preferred.
Cell-free fetal DNA circulates in the plasma of pregnant women (Lo et al., 1997). Up to at least 7% of all cell-free DNA in maternal plasma/serum has been found to be of fetal origin. It has been demonstrated that this fetal DNA can be used to predict the gender of the fetus (Lo et al., 1997), as well as to determine the fetal Rhesus D status (Lo et al., 1998).
Studies have also indicated that the amount of fetal DNA in maternal plasma/serum can be correlated with certain fetal abnormalities such as trisomy 21 (Down syndrome) (Lee et al., 2002) and trisomy 13 (Wataganara et al., 2003). Thus far, the quantitation of fetal DNA has generally relied upon Y-chromosome-specific sequences, which cannot serve to measure the DNA of female fetuses. Accordingly, a gender-independent quantitation of fetal DNA is needed.
There is a need for further methods which identify fetal specific nucleic acids and fetal cell specific markers. In particular, markers that are gender independent are required which can be detected through non-invasive procedures.